Podophyllum peltatum linnaeus (family: Berberidaceae; common names; Ray Apple, American Mandrake, Indian Apple, Wild Lemon, Duck's Foot) is an indigenous North American herbaceous perennial flowering in May. The Indians knew of its properties as a cathartic, anthelmintic and as a poison.
Podophyllin was included in the fourth edition of the United States Pharmacopoeia in 1863 but was dropped from the twelfth edition in 1942. The same year a report by Kaplan appeared that suggested podophyllin was very useful in clinical treatment of a type of venereal wart (Condyloma acuminatum), New Orleans Med. Surg. J., 94: 388 (1942). Between the years 1940 to 1960, podophyllin was studied in many clinical conditions including diseases of the skin due to infectious agents, non-specific dermatoses, gout, rheumatoid arthritis, benign and malignant growth, J. Nat. Cancer Inst., 14: 967 (1954).
A great deal of effort by numerous investigators went into the isolation and identification of the components of podophyllin. In 1880 Podwyssotozki isolated a white crystalline substance which he named podophyllotoxin and which was believed to be the active component. By 1950 seventeen compounds had been isolated and characterized; four were flavonal pigments and the remainder were members of the lignan family. The structures of the compounds isolated are shown in Table 1.
TABLE 1 ______________________________________ COMPOUNDS ISOLATED FROM PODOPHYLLIN NAME # R.sub.1 R.sub.2 R.sub.3 ______________________________________ ##STR5## podophyllotoxin 6 OH H CH.sub.3 .alpha.-peltatin 7 H OH H .beta.-peltatin 8 H OH CH.sub.3 4'-demethylpodophyllo- 9 OH H H toxin deoxypodophyllotoxin 10 H H CH.sub.3 podophyllotoxin 11 Oglucosyl H CH.sub.3 glucoside .alpha.-peltatin glucoside 12 H Oglucosyl H .beta.-peltatin glucoside 13 H Oglucosyl CH.sub.3 4'demethylpodophyllo- 14 Oglucosyl H H ##STR6## picropodophyllotoxin 15 ##STR7## tetradehydropodophyllo- 16 toxin ##STR8## sikkimotoxin 17 ##STR9## quercetin 18 H OH isorhamnetin 19 H OCH.sub.3 quercetin 3-galactoside 20 galactosyl OH kaempferol 21 H H ______________________________________
Podophyllotoxin has antimitotic activity by interacting with microtubules; [Kelly et al, J. Nat. Cancer Inst., 14: 967 (1954); Comman et al, Ann. N.Y. Acad. Sci., 51, 1443 (1952)]. It is also known that colchicine, 22 arrest cells in metaphase; Dustin, Pharmacol. Rev., 15: 449 (1963).
4'-Dimethyl analogues of podophyllotoxin including 4'-demethylpodophyllotoxin, 4'-demethylepipodophyllotoxin, 4'-demethyldeoxypodophyllotoxin and .alpha.-peltatin induce the intracellular degradation of DNA in HeLa cells as well as affecting microtubule formation; Loike et al, Biochemistry, 15: 5443 (1976).
Kuhn and von Wartburg while studying methods of preparing podophyllotoxin glycosides developed a procedure for the preparation of epipodophyllotoxin glycosides which are epimeric to podophyllotoxin at C.sub.4 ; Helv. Chim. Acta. 51: 163 and 1631 (1968) (Scheme 1). ##STR10## They prepared a variety of cyclic acetals of 4'-demethylepipodophyllotoxin B-glycoside and found that they exhibited not only a high activity in vitro but were much less toxic in in vivo testing against mouse lymphocytic leukemia (L-1210); Keller-Jerslen et al, J. Med. Chem., 14: 936 (1971). Two of these derivatives were selected for clinical trials and proved to be very useful in treating a variety of cancers. These were 4'-demethyl-1-O-(4,6-O-ethylidene-.beta.-D-glucopyranosyl)-epipodophylloto xin, 24 (VP-16 or "Etoposide") and 4-demethyl-1-O-(4,6-O-(2-thenylidene)-.beta.-D-glucopyranosyl)-epipodophyl lotoxin, 25 (VM-26 or "Teniposide"). These agents are active in the treatment of a wide spectrum of cancers including bladder, small cell lung, ovarian, thyroid, breast, brain, soft tissue cancers, non-lymphocytic leukemia and Hodgkin's disease; Gensler et al, J. Org. Chem., 31: 3224 (1966).
TABLE 2 ______________________________________ EFFECT OF CYCLIC ACETAL DERIVATIVES ON L-1210 LEUKEMIA MOUSE LEUKEMIA L-1210 R % SURVIVAL TIME INCREASE ______________________________________ CH.sub.3 24 167 CH.sub.2 CH.sub.3 97 ##STR11## 121 136 C.sub.6 H.sub.5 97 C.sub.6 H.sub.5 CH.sub.2 46 1-naphthyl 95 CH.sub.2 CHCH 121 (CH.sub.3).sub.3 C 57 p-FC.sub.6 H.sub.4 64 p-CH.sub.3 C.sub.6 H.sub.4 64 ##STR12## ______________________________________
Stahelin showed that VM-26 and VP-16 prevent cells from entering mitosis and thus unlike the previously mentioned podophyllotoxin analogues they do not arrest cells in metaphase but in late S or G.sub.2 phase of the cell cycle; Oncology, 35: 217 (1978). These compounds surprisingly possess a different mechanism of action other than inhibition of microtubule assembly. In fact, they have been shown not to bind to tubulin; Locke et al, Biochemistry, 15: 5435 (1976). A great deal of effort has gone into determining the exact mechanism of action of these compounds but to data there is no clear answer, although the point of action appears to be the interruption of DNA transcription. Very recent reports suggest that these agents interact with topoisomerase II, an enzyme involved in the uncoiling of DNA supercoils prior to transcription.